A parametric solid modeler is an approach to creating geometric models of 3D objects and scenes. Constructive Solid Geometry (CSG) and Boundary Representation (Brep) methods are two fundamental solid modeling methods. CSG uses solid primitive shapes (cones, cylinders, torus, spheres, rectangular prisms, etc.) and boolean operations (unions, subtractions, intersections) to create a solid model. A cylinder subtracted from a cube produces a hole, for instance. Brep methods start with one or more wireframe profiles, and create a solid model by extruding, sweeping, revolving or skinning these profiles. The boolean operations can also be used on the profiles themselves and the solids generated from these profiles. Solids are also created by combining surfaces through a sewing operation. Most commercial solid modeling systems are hybrids combining both CSG and Brep methods to create the desired models.
In a parametric model, each geometric entity has parameters associated with it. These parameters control the various geometric properties of the entities, such as the length, width and height of a rectangular prism, or the radius of a fillet. They also control the locations of these entities within the model. Parameters are changed by the user to create a desired model.
If the task is to construct a 3D model of a scene or object depicted in a sketch or a photograph, the use of a conventional solid modeling system can be an arduous task. This is especially true when producing a texture-mapped model, where the original photograph or other imagery is applied to the derived geometric model. The current process typically involves first creating a 3D geometric model then tediously “cutting and pasting” textures onto the model to add realism. Ultimately, the results are limited by the hand-eye coordination of the user. In short, the current approach and available tools cannot achieve desired levels of geometric accuracy, realism (visual fidelity), are too time-consuming, and require too much skill.